Gaseous diffusion technique

ABSTRACT

A counter-flow of an inert, or a predominantly inert, gas is provided in a diffusion furnace to provide means for creating turbulence within the diffusion system thereby improving the uniformity of dopant along the length of the diffusion carrier contained therein.

O United States Patent 1151 3,660,179 Desmond et al. 1 May 2, 1972- 54GASEOUS DIFFUSION TECHNIQUE 3,314,393 4 1967 Haneta ..118/48 3,354,00411/1967 Reisman et a1.. 17/106 A X [721 g z' 'z i' zfg gg i'ifl fi g fig} 3,361,600 1/1968 Reisman et a1 ..117/106 A x f n e S 3,517,643 6/1970GOlClSlell'l et a1 ..118 48 3,594,242 7/1971 Burd et a1 ..252/6236 A X[731 Assignec: Westinghouse Electric Corporation, Pitt- Sburghv PrimaryExaminer-Tobias E. Levow 2 Filed: 17 970 Assistant Examiner-J. CooperAttorney-F. Shapoe and C. L. Menzemer 211 Appl. No.: 64,381

I [57] ABSTRACT [52] g A counter-flow of an inert, or a predominantlyinert, gas is l t Hon M4 provided in a diffusion furnace to providemeans for creating [58] g i 1 18/48 turbulence within the diffusionsystem thereby improving the l 49 252/62 3 i 62 3 uniformity of dopantalong the length of the diffusion carrier contained therein.

[56] References Cited 8 Claims, 1 Drawing Figure UNITED STATES PATENTS3,021,198 2/1962 Rummel ..117/106 A X S ECOND GAS EOUS STREAM PATENTEDmzI972 3,660,179

FIRST GASEOUS SECOND STREAM Q GASEOUS STREAM WITNESSES n INVENTORS VTimothy J. Desmond 0nd I BEerton P. Krumonucker GASEOUS DIFFUSIONTECHNIQUE BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to the processing of semiconductor materials and inparticular to the diffusion of a dopant materialinto a body ofsemiconductor material.

2. Description of the Prior Art Heretofore the process of diffusing adopant material into semiconductor material comprised a flow of gasincluding a specified: proportion of dopant material in a gaseous phasewas continuously passed through a furnace over and about a series ofheated bodies of semiconductor material, the gaseous dopant materialbeing introduced through one end of the furnace and exiting from theother end of the furnace. Invariably, the flow of the gas was relativelysmooth. Unfortunately, it has been found that this results innon-uniform doping of the bodies not only from one end of the furnace tothe other but often on different portions of the exposed faces of anyone individual body.

SUMMARY OF THE INVENTION In accordance with the teachings of thisinvention, there is provided a process for the uniform gaseous diffusionof a doping material into exposed surfaces of a body of semiconductormaterial. Briefly, the process comprises supporting within a generallyelongated furnace the body of semiconductor material spaced from thebottom of the furnace. The body is heated to an elevated temperature atwhich diffusion of dopant can occur, and a first gaseous streamcomprising, in part, a dopant material is introduced into the furnacethrough an inlet tube at one end of the furnace. A second gaseous streamhaving a flow rate greater than the first stream is introduced at theother end of the furnace at a distance from said body and its support.The second gaseous stream is directed initially to flow between the bodyand the bottom of the furnace, striking the end of the furnacecontaining the inlet tube thereby reversing its flow direction andcausing the two gaseous streams to interrnix in a turbulent manner,thereby depositing dopant material on, and diffusing into the exposedsurfaces of the heated body of semiconductor material and exhausting theremainder of the intermixed gaseous streams through the other end of thefurnace. A plurality of bodies, such as wafers of silicon can be treatedin the furnace. A much higher degree of uniformity of doping of theexposed surfaces of all the bodies is thereby secured.

DRAWING The FIGURE shows, in part, a cross-section of a furnaceconstructed and operating in accordance with the teachings of thisinvention.

DESCRIPTION OF THE INVENTION With reference to the FIGURE there is shownfurnace apparatus l0 suitable for diffusing semiconductor materials witha suitable dopant material in accordance with the teachings of thisinvention. The furnace 10 comprises a tube 12 having an inlet port 14located centrally at one end thereof for introducing a stream of gaswhich contains a suitable dopant material in vapor form into the tube12.

One or more bodies 16 of semiconductor material, for example, wafers ofsilicon are supported in a boat 18 supported by legs 19 or if wideenough by the walls of the tube 12, so that the boat is spaced adistance above the tube bottom. The bodies 16 are heated to thediffusing temperature by a resistance coil heater 20 encircling the tube12. A gas inlet 22 is inserted into the other end of the tube 12 and itsopen end is disposed on the bottom surface of the tube 12. Through theinlet 22 is introduced a counterflow of an inert, or a predominantlyinert, gas to create a desired high turbulence in the stream of dopantgas introduced by port 14 within the tube 12. The flow rate of the gasinjected into the furnace through the inlet 22- must be greater than theflow rate of the dopant gas from port 14. Preferably the ratio of theflow rate of counterflow gas to dopant gas is of the order of 2: I. Ifthe open end of inlet 22 is placed within several inches from the boat18 the best results are obtained. It has been found that less pronouncedbenefits are secured obtaining a more uniform distribution of dopantmaterials in the semiconductor materials is achieved if the end of inlet22 is at the end of the boat or if it is more than 6 to 8 inches fromthe end of the boat.

When no counterflow gas flows from inlet 22, it appears that when thedopant gas enters through the inlet port 14 into the near end of thefurnace 12, some turbulence may occur immediately around the port 14 butthe dopant gas achieves essentially a laminar flow within a few inchesfrom the inlet port 14 and retains such laminar flow condition by thetime it reaches the end of the boat 18 closest to the port 14. Uponreaching and striking the end of the boat 18 closest to the port 14 thelaminar pattern of the dopant gas stream is broken up partly, with thatportion of the dopant gas stream directly striking the boat 18 beingdeflected and becoming turbulent thereby tumbling over and about thebodies 16. Some disturbance of the portion of the laminar flowing dopantstream nearer the boat also occurs. The outermost portions of the gasstream appear to be less disturbed and retain most of their laminar flowpattern. After passing the boat 18 the dopant gas flow then assumes arelatively smooth laminar flow pattern and exits at a large outlet 24the far end of the tube 12.

When the inert gas is introduced into the tube 12 through the inlet 22inserted through the far end of the furnace, it appears that the inertgas, flowing not only at a greater flow rate than the dopant gas butcountercurrent thereto rapidly breaks up the laminar flow pattern of thedopant gas in the bottom of the tube 12, particularly at the rear of andbeneath the boat 18 thereby causing a turbulent gas flow pattern tooccur. For this reason the inlet 22 is designated hereinafter as atubular tube. Some of this turbulent gas stream mixes with the turbulentdopant gas stream flow immediately around the top of the boat 18 toprovide a better bathing of the exposed surfaces of each individual body16 to the gaseous dopant material. Additionally, since the gas flow rateof the gas introduced through the turbulator exceeds that of the dopantgas flow rate, the flow beneath the boat 18 towards the end of the tube12 where port 14 is located creates a venturic effect which tends toforce more of the dopant gas stream in turbulent fashion down past andabout the bodies 16 from the top of the boat 18 to the bottom. The mainstream of gas from the turbulator tube 22, in general, then continues tothe end of the tube 12 at port 14, striking the wall and flowingupwardly so that it mixes with the dopant gas entering the tube 12through inlet port 14, creating I considerable turbulence and mixingthereof, and assumes only partial laminar flow configuration beforestroking the boat 18 and the bodies 16, becoming turbulent again andflowing about the bodies 16 and the boat 18, some of which is capturedinto fresh inert gas continually flowing beneath the boat 18, resumeslaminar flow configuration after passing the boat 18, and exits from thetube 12 through the far end at outlet 24. Additionally, the gasintroduced from the turbulator tube 22 may create a pressure conditionwhich enables the dopant gas to dwell longer around the bodies 16 longerthan previously obtainable. Regardless of our explanation, the neteffect of the apparatus and process of this invention is the achievementof better uniformity and reproducibility of the diffusion therebyessentially duplicating the optimum results obtained with the sealedtube diffusion process, the latter process being more expensive anddifficult to operate.

The same effect may be achieved by using a gas emerging from the tube 22at a greater velocity than the initial dopant gaseous stream. Thevelocity of this stream of gas must be of such magnitude as to carryessentially completely past the underside of the boat 18 to accomplishthe venturi effect on the gas bathing the wafers 16 from the top of boat18 before this stream which has entered from tube 22 is intermixed withthe gas stream from tube 14.

The inert gas is a gas which is completely or relatively chemicalunreactive with the dopant gases and the bodies used in the processpracticed with apparatus 10. However, a portion of the dopant gasrequired for the overall diffusion process could be introduced into thefurnace in the counterflow gas stream through the turbulator tube 22.

Examples of the use of this apparatus and process of this invention areasfollows:

A 60 mm. ID flow through tubular furnace was prepared for the diffusionof wafers of silicon semiconductor material. Five 1 12inch diameterwafers of silicon semiconductor material were lapped, polished andcleaned and placed in a boat which in turn was disposed in the furnacewith a space below the boat. The boat measured 10 inches in length by linches in width. The wafers were all parallel to the length of the tube.One of the wafers was placed in each of the corners of the boat and thefifth wafer (wafer No. 3) was placed in the center of the boat.

Thewafers were heated to a temperature of 1,000 C. i l C. by aresistance type heater encircling the furnace 12 in the vicinity of thewafers. A gas stream consisting essentially of 4,300 cc./min. ofnitrogen, l80 cc./min. of oxygen, and 9 cc./min. of a gaseous mixture of2 percent diborane in nitrogen was introduced into the furnace. The gasstream was caused to flow through the furnace and over and about thewafers for 30 minutes.

When a quartz turbulator tube of 9 mm. ID was employed, having itsopening 3 inches from the end of the boat the inert gas was nitrogenflowing at a specific flow rate indicated below for each run, the inertgas flow being counter to that of the direction of dopant gas mixture.All measurements were measured with a four point probe and a constantelectrical current of l milliampere. The sheet resistance is obtained bymultiplying the reading in millivolts vby 4.5. Readings were taken oneach side of each wafer.

The sheet resistance of the wafers as determined from the test resultswere erratic and varied from each other quite a bit by a factor offour,as well as showing an unacceptable variance betweeneach side of the samewafer for wafers 2, 3, 4 and 5. The sheet resistance of the wafersvaried too greatly to be acceptable for making production runs ofsemiconductor devices of uniform characteristics. In addition to thisnonuniformity of sheet resistance readings, all wafers had a bluishoxide coating formed only onvthe edge area of each 'wafer nearest thedopant inlet port.

EXAMPLE I! With Turbulator Tube-Nitrogen Gas Flow Rate 25 L/min.

Wafter Side Millivolts l a 27.00 b 23.00 2 a 24.90

The ratio of the turbulator tube gas flow to dopant gas flow is about5:1. These wafers showed a great improvement in the uniformity of thesheet resistance as determined from the test results for each side 'ofeach wafer as well as from wafter to wafer. None of the wafers werediscolored by the bluish oxide coating noted on the wafers of Example 1.However, the turbu- The ratio of turbulator tube to dopant gas flow isabout 2.5 to 1. The sheet resistance as determined from the test resultsfor each wafer varies very little from side to side compared to previousresults obtained and also each wafer does not differ much from thebalance of the wafers in the test boat. Additionally, the overall effectof the lower flow of nitrogen gas introduced through the turbulator tubeis to produce wafers having low sheet resistance values, consequentlygood doping has been obtained. This is very desirable for semiconductordevice applications.

To Turbulator Tube-the effect of temperature in the process thefollowing tests were made: I

EXAMPLE IV Temperature of Wafers 950 C ;t l C.

With TurbulatorNitrogen Gas Flow Rate 10.4 l./min.

, The sheet resistance values as determined from the test results varygreatly from side to side on each wafter and from 7 wafer to wafer inthe boat. The sheet resistance values have also increased with adecrease in temperature indicating lesser dopant being deposited on thewafer:

EXAMPLE V Temperature of Wafers 1050 C. 1 1 C.

With Turbulator Tube-Nitrogen Gas Flow Rate 10.4 l./min.

Wafer Side Millivolts The sheet resistance values as determined from thetest results have dropped to a lower value being of the order ofone-eighth the value obtained at 950 C. But the sheet resistance valuesare very uniform from side to side on each wafer as well as from waferto wafer.

EXAMPLE VI 2% Diborane Gas Flow-25 cc./min. Temperature of Wafers 1050C. i 1 C.

With Turbulator Tube-Nitrogen Gas Flow Rate 10.4 l./min.

Wafer Side Millivolts When compared to the results obtained in ExampleVI, the sheet resistance values obtained in this experimental run are alittle more uniform both in side to side comparison for each wafer andfrom wafer to wafer. However, the increase in dopant concentration hasalso increased the sheet resistance values of each wafer slightly.

The results of Examples V and V] are similar to the best resultsobtained from closed tube diffusion systems. From the results obtainedfrom the experimental runs it has been found that the turbulator tubeapplication for introducing a counterflow of an inert gas into the opentube diffusion process system will, under proper temperature conditions,essentially duplicate the closed tube diffusion process, but at a greatsaving of time and expense. The sheet resistivity values obtained fromthe test results of the experiments indicate that too great acounterflow of inert gas beyond a 2:1 to 3:1 range is less desirable ifuniformity of the sheet resistance values of all wafers in oneproduction run is required. The sheet resistivity values for a giventime for diffusion can be increased by lowering the temperature of thewafer or increasing the volume of diborane gas flow or both. An increasein temperature of the wafers being treated produces a lower resistivityin the wafers. The flow rate of nitrogen for the turbulator tube to thedopant gas flow should preferably not exceed about a 4:1 ratio.

As mentioned previously the location of the turbulator tube in thefurnace in respect to the boat containing the wafers is a factor forbest results. The inert gas flow exit from the turbulator tube in theorder of from 3 to 4 inches from the end of the boat for the dimensionsof the furnace, the turbulator tube,

and the boat described in the Examples, has given excellent results.

The gaseous stream introduced through the turbulator tube need not beentirely inert. Part of the gaseous stream may comprise a dopantmaterial, preferably the same as in the first gaseous dopant stream. inthe instance of doping silicon semiconductor material with boron from.diborane a small amount of oxygen may also be included in the secondgaseous stream.

Although the invention has been taught specifically for a round tubularpass through furnace, the process application is suitable for furnacesof other shapes provided the two gas streams are counterflow to eachother and the one stream of greater velocity and/or of greater volume isdirected initially below the wafers being doped.

We claim as our invention:

l. A process for the uniform gaseous diffusion of a dopant into a bodyof semiconductor material comprising:

a. supporting within a tubular furnace said body of semiconductormaterial spaced from the bottom of the furnace; b. heating said body ofsemiconductor material to a temperature at which the dopant diffusesinto said body; I

c. introducing a first gaseous stream comprising, a dopant materialthrough an inlet port in one end of the furnace into the furnaceconfines;

d. introducing a second gaseous stream comprising an inert gas throughthe other end of the furnace into the furnace confines, at apredetermined distance from said body support, said gaseous streamhaving a flow rate greater than the flow rate of the first gaseousstream;

e. directing the flow of said second gaseous stream to flow beneath thebody of semiconductor material between said body and said bottom of thefurnace, thence to strike the end of the furnace containing the inletport therein to change the direction of the flow of said second gaseousstream thereby reversing its flow direction and causing said first andsecond gaseous stream to intermix in a turbulent manner;

diffusing said dopant material from said intermixed gases in exposedsurfaces of said heated body of semiconductor material; and

g. exhausting the remaining portions of said first and said secondgaseous streams through said other end of the said furnace.

2. The process as defined in claim 1 in which said inert gas is oneselected from the group consisting of nitrogen and argon.

3. The process as defined in claim 2 wherein said second gaseous streamfurther contains, a dopant material.

4. The process as defined in claim 1 wherein said second gaseous streamhas a flow rate of at least twice the flow rate of said first gaseousstream.

5. The process as defined in claim 1 wherein the first gaseous streamcomprises oxygen, nitrogen and a doping gas mixture of dibrane andnitrogen and the second gaseous stream comprises nitrogen.

6. The process as defined in claim 5 wherein the second gaseous streamfurther contains oxygen, and

diborane.

7. The process as defined in claim 5 wherein the flow rate of the oxygenis cc./min., the flow rate of nitrogen of the first gaseous stream is4,300 cc./min., the flow rate of the diborane doping gas mixture is 9cc./min., the flow rate of nitrogen in the second gaseous stream is nogreater than 15 l./min., the furnace has an inside diameter of 60 mm.ID, said predetermined distance is from 3 to 4 inches, and saidsemiconductor material is'silicon.

8. The process as defined in claim 7 wherein the flow rate of nitrogenin said second gaseous stream is 10.4 l./min.

2. The process as defined in claim 1 in which said inert gas is oneselected from the group consisting of nitrogen and argon.
 3. The processas defined in claim 2 wherein said second gaseous stream furthercontains, a dopant material.
 4. The process as defined in claim 1wherein said second gaseous stream has a flow rate of at least twice theflow rate of said first gaseous stream.
 5. The process as defined inclaim 1 wherein the first gaseous stream comprises oxygen, nitrogen anda doping gas mixture of dibrane and nitrogen and the second gaseousstream comprises nitrogen.
 6. The process as defined in claim 5 whereinthe second gaseous stream further contains oxygen, and diborane.
 7. Theprocess as defined in claim 5 wherein the flow rate of the oxygen is 180cc./min., the flow rate of nitrogen of the first gaseous stream is 4,300cc./min., the flow rate of the diborane doping gas mixture is 9cc./min., the flow rate of nitrogen in the second gaseous stream is nogreater than 15 l./min., the furnace has an inside diameter of 60 mm.ID, said predetermined distance is from 3 to 4 inches, and saidsemiconductor material is silicon.
 8. The process as defined in claim 7wherein the flow rate of nitrogen in said second gaseous stream is 10.4l./min.